Optical disk memory as high-density and high-capacity memory, such as a read-only CD (compact disk) and a magneto-optical disk which allows information recorded thereon to be rewritten, has already been on the market.
As illustrated in FIG. 9, read-only memory has minute pits 42 as permanent information created in spiral form on one of the surfaces of a transparent substrate 41 such PC (polycarbonate) and PMMA (acrylic) substrates. Such a disk having the pits allows the mass production of the same information.
On the other hand, as illustrated in FIG. 10, a magneto-optical disk has spiral guide grooves 52 on one of the surface of a transparent substrate 51, and a recording medium designed for use with a magneto-optical disk on the surface of the transparent substrate 51 whereupon the guide grooves 52 are formed. With this magneto-optical disk, signals are recorded on and reproduced from the recording medium. The location on which signals are to be recorded is either on the guide grooves 52 or lands 53 between the guide grooves 52 (if the width of the land 53 is larger than that of the guide groove 52 as shown in FIG. 10, signals are usually recorded on the lands 53). Since the recording medium is a film of a magnetic substance, it is possible to erase the recorded information and to record new information over the erased information. This is a significant characteristic of a magneto-optical disk.
A magneto-optical disk having both the characteristics of the rewritable disk and the disk containing only the permanent information is proposed. Such a disk with ROM (read-only memory) is divided into zones at a predetermined radius of the disk. For example, as illustrated in FIG. 11, a magneto-optical disk is divided into zone A where the permanent information is recorded in the form of pits and zone B where guide grooves are formed and rewritable information is recorded.
However, with the structure of the conventional magneto-optical disk with ROM, the ROM area where the permanent information is recorded and the magneto-optical memory area where information is rewritable are merely provided. Therefore, the volume of information recorded on the disk remains the same. Moreover, when reading the permanent information recorded in the ROM area and recording new information in the magneto-optical area according to the content of the information, it is necessary to move a light beam from the inner zone A toward the outer zone B.
In order to reproduce signal from an optical disk, generally, an optical pickup is required. As illustrated in FIG. 12, the optical pickup includes lens systems 72 and 77 for converging light 78 from a light source 71 such as a semiconductor laser, a photodetector 75 for reading information signals, a beam splitter 73 for directing reflected light from the disk to the photodetector 75, and a lens 74.
With this structure, however, since the weight of the optical pickup is increased, it takes at least several tens ms to move the optical pickup from the center portion of the optical disk toward the outer portion, resulting a linger access time to the information.
In short, the structure of the conventional magneto-optical disk with ROM suffers from the following drawbacks: the volume of information recorded on a single disk is the same as that recorded on a disk having only the ROM area or disk having only the magneto-optical area; and it takes a long time to access to the ROM area and the magneto-optical area by turns.